Strictly non-blocking optical switch core having optimized switching

ABSTRACT

A switch core is set forth that comprises a plurality of duplex switches that are interconnected with a interconnection fabric to implement, for example, strictly non-blocking operation of the switch core for reciprocal traffic. In one embodiment, an N-way reciprocal switch is implemented. The N-way reciprocal switch comprises a plurality of duplex switches numbering N of at least a 1×(N−1) switch type (e.g., the duplex switches have at least N−1 ports available for connection to implement the interconnection fabric). The interconnection fabric interconnects the plurality of duplex switches so that each duplex switch is connected to every other duplex switch used in the interconnection fabric by a single connection. A similar architecture using switches numbering N of at least a 1×N switch type are also set forth. Still further, a plurality of duplex switches are used to implement an (n,m)-way switch that, in turn, can be used to construct a recursive LM-way switch core and/or recursively expand an existing Clos switch core.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation application of U.S. patent applicationSer. No. 09/143,335, filed Sep. 4, 1998, entitled STRICTLY NON-BLOCKINGOPTICAL SWITCH CORE HAVING OPTIMIZED SWITCHING ARCHITECTURE BASED ONRECIPROCITY CONDITIONS.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable

BACKGROUND OF THE INVENTION

[0003] Optical switches and switching architectures are used in opticalnetworks for a variety of applications. One application of opticalswitches is in provisioning of light paths. In this application, theswitches are used to form optical cross-connect architectures, which canbe readily reconfigured to support new light paths. In this application,the switches are replacements for manual fiber patch panels. As such,switches with millisecond switching times are acceptable. The challengewith respect to such applications is to realize large switch sizes.

[0004] At the heart of the optical switch is the switch core. In termsof switching function, switch cores may be characterized as eitherblocking or non-blocking architectures. A switch core architecture issaid to be non-blocking if any unused input port can be connected to anyunused output port. Thus a non-blocking switch core is capable ofrealizing every interconnection pattern between the inputs and theoutputs. If some interconnection patterns cannot be realized, the switchis said to be blocking. A popular architecture for building largenon-integrated switch cores is the Spanke architecture illustrated inFIG. 1. In accordance with the Spanke architecture, an N×N switch ismade by combining N switches of the 1×N switch type along with Nswitches of the N×1 switch type, as illustrated. The Spanke architectureresults in a strictly non-blocking switch core architecture thatrequires 2N switches. The switch illustrated in FIG. 1 is a 4×4 switchcore.

[0005] The increasing popularity of optical networks has resulted in theneed for larger optical switch cores, thereby increasing the number ofinput and output channels (N). Since, in accordance with the formulaabove, the total number of switches used as well as the size of eachswitch in the Spanke switch core architecture increases substantially asthe number of input and output channels increases, the cost of providinga large switch is significant and, in some instances, prohibitive.

[0006] The present inventors have recognized the reciprocal nature ofthe connections in a typical optical switch core employed in aconventional optical network. These reciprocity conditions have beenused by the present inventors to provide a strictly non-blocking opticalswitch core architecture that significantly reduces the number ofswitches that are required to construct the switch core.

BRIEF SUMMARY OF THE INVENTION

[0007] A switch core is set forth that comprises a plurality of duplexswitches that are interconnected with an interconnection fabric toimplement, for example, strictly non-blocking operation of the switchcore for reciprocal traffic. In one embodiment, an N-way reciprocalswitch is implemented. The N-way reciprocal switch comprises a pluralityof duplex switches numbering N of at least a 1×x(N−1) switch type (e.g.,the duplex switches have at least N−1 ports available for connection toimplement the interconnection fabric). The interconnection fabricinterconnects the plurality of duplex switches so that each duplexswitch is connected to every other duplex switch used in theinterconnection fabric by a single connection. Such an architecture mayalso be used to implement a switch that is not strictly non-blocking.

[0008] In a second embodiment, an LM multi-stage reciprocal switch corehaving recursive properties and corresponding (n,m)-way switches are setforth. The LM multi-stage reciprocal switch core is comprised of aplurality of M-way reciprocal switches numbering at least 2L−1. Each ofthe plurality of M-way reciprocal switches is implemented as an N-wayreciprocal switch described above, where N=M. A plurality of(L,2L−1)-way reciprocal switches numbering M are also used. Themulti-stage LM reciprocal switch is itself an LM-way reciprocal switchthat can be used to recursively build larger switches. For example, theLM reciprocal core switch can be used to implement a larger L₁M₁multi-stage switch in which M₁=LM. Alternatively, or in addition, theM-way switches used to build the LM switch core can also be multi-stagein nature and built from smaller recursive components; i.e., from(j,2j−1)-way switches and (M/j)-way switches.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0009]FIG. 1 is a schematic block diagram of a Spanke switch.

[0010]FIG. 2 is a schematic block diagram of a non-square rectangularSpanke switch core.

[0011]FIGS. 3A and 3B are schematic block diagram of N-way reciprocalswitches constructed in accordance with the present invention.

[0012]FIG. 4 is a block diagram of a 1×k duplex switch.

[0013]FIGS. 5A and 5B are block diagrams of 1×k duplex switchesconstructed from smaller order duplex switches.

[0014]FIGS. 6A and 6B are schematic block diagrams of variousembodiments of (n,m)-way reciprocal switches constructed in accordancewith the present invention.

[0015]FIG. 7 is a schematic block diagram of an LM×LM non-blocking, Closswitch core.

[0016]FIG. 8 is a schematic block diagram of an LM port reciprocalswitch core constructed in accordance with the teachings of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] In the Spanke switch architecture illustrated in FIG. 1 for anN×N switch, there are two columns of switches: a left input column of1×N types switches, shown generally at 10, and a right output column ofN×1 type switches, shown generally at 15. The column of switches 10function as input ports that accept external traffic and direct thattraffic through the interconnection fabric, shown generally at 17, whilethe column of switches 15 function as output ports that provide theswitched traffic to an external device.

[0018]FIG. 1 also illustrates the case where reciprocity exists for apath connecting the “eastbound” signal at input port 20 to the outputport at 25. In accordance with this reciprocity condition, thecorresponding “westbound” signal at input port 30 is connected to theoutput port 35. In a more general sense, reciprocity exists if inputport A connects its traffic to output port B whenever input port Bconnects its traffic to output port A.

[0019] The present inventors have recognized that, under such reciprocalpath conditions, the port position used by an input switch in the leftinput column 10 to direct the traffic through the interconnection fabricdirectly corresponds to the interconnection fabric port position of thecorresponding output switch of the right output column 15 (e.g., fabricport 4 of input switch 20 is provided to the output switch 25 which isin the fourth position of the right output column 15, while fabric port1 of input switch 30 is provided to output switch 35 which is in thefirst position of the right output column 15).

[0020] The reciprocity condition has several interesting consequences.In the switch core of FIG. 1, switch 20 and switch 35 are doing the samething: they are both set on fabric port position 4. The presentinventors have recognized that this means that the switches 20 and 35can be implemented as the same physical switch with separate beams oflight passing in opposite directions through common lenses or mirrorsdisposed inside the switch. As such, depending on the specificconstruction of the individual switches, the same actuators, mirrors,lenses, etc., that constitute the left-to-right connection through inputswitch 20 can be duplicated to carry a right-to-left connection byemploying a second set of optics in which the second set of opticsconstitutes the output switch 35. In some cases, a second set of opticsis not needed. In such instances, the same set of optics can carry twoparallel beams going in opposite directions. In effect, the input andoutput switches are collapsed into a single 1×N duplex switch having twobeams of light carrying traffic in opposite directions. For onedirection, such a duplex switch is acting like an input 1×N switch, andfor the other direction as an output N×1 switch.

[0021] A square N×N switch architecture is not the only type ofarchitecture in which reciprocity may exist and used to an advantage.FIG. 2 illustrates a non-square rectangular Spanke switch core in whichnot all inputs are connected to all outputs. Unused paths are indicatedby dashed lines. Such a switch, however, can still take advantage of thesavings associated with reciprocity. In a reciprocal connectioncondition, input switch 40, as above, is sending the “eastbound” trafficthrough the fabric 17 from the fabric port at position 4 to the fabricport at position I of the output switch 45 in the fourth position.Likewise, input switch 50 sends the corresponding “westbound” trafficthrough the fabric 17 from the fabric port at position I to the fabricport at position 4 to the output switch 55 in the first position. Onceagain, such reciprocity means that each input switch can be combinedwith the corresponding output switch by means of double light pathsthrough common lenses or mirrors. The principal difference between theswitch core of FIG. 1 and the switch core of FIG. 2 is that certainpaths in the core of FIG. 2 are not utilized as indicated by the thindashed lines.

[0022] Since much of the cost of most switches typically centers on theactuation mechanism and mirror or lens employed in the switch, using aswitch that is collapsed so that these components are common to bothlight paths can approach a 2-to-1 savings, provided that the paths arereciprocal. It has been found, that the typical networks, such as SONETrarely, if ever, violate this reciprocal condition.

[0023] Application of the foregoing principles to design large opticalswitch cores results in a number of different switch core architecturesthat are optimized when compared to their traditionally designed switchcore counterparts. The optimized switch core architectures are comprisedof one or more stages of duplex switch modules, such as the singlemodule shown at 60 of FIG. 3. Each duplex switch module 60 is comprisedof individual 1×k duplex switches, such as at 65 of FIGS. 3A and 4,where k may vary from switch to switch within the module 60. As notedabove, a 1×k duplex switch generally functions as a traditional 1×kswitch, but allows signal traffic to flow in both directions of theswitch thereby allowing the switch to function as both an input andoutput switch sharing common optical components for the input and outputpaths.

[0024] One embodiment of a switch core architecture that uses theforegoing principles to reduce the complexity of the switchingarchitecture is illustrated in FIG. 3A. As illustrated, the opticalswitch core includes a single N-way duplex module 60 comprised of Nswitches 65 of the 1×(N−1) duplex switch type (i.e., k=N−1). Such aswitch core 60 allows duplex connections between any pair of free portsregardless of existing connections and, as such, is similar to the N×Nstrictly non-blocking Spanke switch architecture of FIG. 1. However,switch core 60 is only strictly non-blocking for reciprocal traffic.

[0025] In the embodiment shown in FIG. 3A, module 60 is a 4-wayreciprocal switch core , as such, uses 4 duplex switches of the 1×3switch type. The 1×3 duplex switches interconnected to form the fabricof the 4-way reciprocal switch core in the manner forth in Table 1.TABLE 1 Switch Fabric Position Port Internal Port Connection 1 1 FabricPort 1 Of Switch at Switch Position 2 1 2 Fabric Port 1 Of Switch atSwitch Position 3 1 3 Fabric Port 1 Of Switch at Switch Position 4 2 1Fabric Port 1 Of Switch at Switch Position 1 2 2 Fabric Port 2 Of Switchat Switch Position 3 2 3 Fabric Port 2 Of Switch at Switch Position 4 31 Fabric Port 2 Of Switch at Switch Position 1 3 2 Fabric Port 2 OfSwitch at Switch Position 2 3 3 Fabric Port 3 Of Switch at SwitchPosition 4 4 1 Fabric Port 3 Of Switch at Switch Position 1 4 2 FabricPort 3 Of Switch at Switch Position 2 4 3 Fabric Port 3 Of Switch atSwitch Position 3

[0026] In accordance with the foregoing interconnections of the duplexswitches of the 1×(N−1) switch type, each switch is connected to everyother switch by a single fabric interconnection. Many other permutationsare possible for interconnecting the switches. The principal criterionis to connect each switch to every other switch.

[0027] If 1×4 duplex switches (e.g., 1×(N) type duplex switches) areused, a strictly non-blocking switch architecture having loop-back maybe implemented. Such an architecture is illustrated in FIG. 3B.Interconnections between the duplex switches in such an architecture areas set forth in Table 2. TABLE 2 Switch Internal Position Port PositionInternal Port Connection 1 1 Loop-back 1 2 Internal Port 1 Of Switch atSwitch Position 2 1 3 Internal Port 1 Of Switch at Switch Position 3 1 4Internal Port 1 Of Switch at Switch Position 4 2 1 Internal Port 2 OfSwitch at Switch Position 1 2 2 Loop-back 2 3 Internal Port 2 Of Switchat Switch Position 3 2 4 Internal Port 2 Of Switch at Switch Position 43 1 Internal Port 3 Of Switch at Switch Position 1 3 2 Internal Port 3Of Switch at Switch Position 2 3 3 Loop-back 3 4 Internal Port 3 OfSwitch at Switch Position 4 4 1 Internal Port 4 Of Switch at SwitchPosition 1 4 2 Internal Port 4 Of Switch at Switch Position 2 4 3Internal Port 4 Of Switch at Switch Position 3 4 4 Loop-back

[0028] It will be recognized in view of the foregoing description thatother permutations for the interconnect fabric are also possible. Theprincipal goal is to connect each switch to every other switch by atleast a single fabric interconnection. The specific interconnections ofthe duplex switches of the 1×N switch type in Table 2, however, can begeneralized in the following manner. Let X represent the switch positionof the duplex switch in the overall switch architecture, where X is anumber from, for example, 1 through N. Let Y represent the fabric portof switch X, where Y is a number from, for example, 1 through N. Tointerconnect the duplex switches to form a strictly non-blocking, N-wayswitch for reciprocal traffic, each fabric port Y of each switch X isconnected to the fabric port X of switch Y when X≠Y, and wherein eachpath Y may optionally be used for loop-back when X=Y. Again, suchinterconnections are made starting with switch X=1 until each duplexswitch is connected to every other duplex switch of the interconnectionfabric by a single connection.

[0029]FIGS. 5A and 5B illustrates various manners in which duplexswitches of a lesser order may be cascaded to form larger 1×k duplexswitches, such as the one shown at 65 of FIG. 4. More particularly, FIG.5A illustrates a 1×12 reciprocal switch at 65 that is comprised of asingle 1×3 reciprocal switch 67 that is cascaded with a further group ofthree 1×4 reciprocal switches 69. In like fashion, FIG. 5B illustrates a1×9 reciprocal switch at 65 that is comprised of a single 1×3 switch 71that is cascaded with a further group of three 1×3 reciprocal switches73. It will be recognized that other 1×k reciprocal switches may beformed from lesser order reciprocal switches. In such instances, theoverall switching architecture may be optimized by using as few of thelower order or reciprocal switch types as possible, thereby reducing thenumber of component types required to manufacture the overall switch.

[0030] Other switch architectures may be implemented in accordance withthe foregoing principles. One such architecture is the (n,m)-way module,shown generally at 80 of FIG. 6A. The (n,m)-way module 80 is similar infunctionality to the n-way module 60 of FIG. 3, except that it has n+mduplex switches. The duplex switches are logically divided into twogroups: a first group of switches 85 numbering n and a second group ofswitches 90 numbering m. Only the first group of switches 85 can formduplex connections to any other port in the module 80. The second groupof switches 90 can only connect to the switches of the first group 85.

[0031] In the preferred construction of the (n,m)-way module 80, a totalof n duplex switches 95 of the 1×(n+m−1) type are employed for the firstgroup of switches 85 and a total of m duplex switches 100 of the 1×ntype are employed for the second group of switches 90. To effect thestated operation of the first group of ports 85, the fabric ports ofeach switch 95 of the first group of switches 85 are connected to thefabric ports of every other switch in the module 80. This insures thatthe first group of switches 85 is allowed to form duplex connections toany other switch in the module 80. To effect the stated operation of thesecond group of switches 90, each fabric port of each switch 100 in thesecond group of switches 90 is connected only to a respective fabricport of the first group of switches 95. As such, each duplex switch ofthe first group of switches 85 is connected to every other switch by asingle interconnection, while each duplex switch of the second group ofswitches 90 is interconnected to each of the first group of switches 85by a single interconnection without further interconnection to any ofthe switches 100 of the second group.

[0032] The exemplary module 80 of FIG. 6A illustrates construction of a(3,4)-way module. Interconnections between the duplex switches in theillustrated architecture are as set forth in Table 3. Again, variouspermutations may be employed. TABLE 3 Switch Fabric Position PortPosition Fabric Port Connection First Group—1 1 Fabric Port 1 Of FirstGroup Switch at Switch Position 2 First Group—1 2 Fabric Port 1 Of FirstGroup Switch at Switch Position 3 First Group—1 3 Fabric Port 1 OfSecond Group Switch at Switch Position 1 First Group—1 4 Fabric Port 1Of Second Group Switch at Switch Position 2 First Group—1 5 Fabric Port1 Of Second Group Switch at Switch Position 3 First Group—1 6 FabricPort 1 Of Second Group Switch at Switch Position 4 First Group—2 1Fabric Port 1 Of First Group Switch at Switch Position 1 First Group—2 2Fabric Port 2 Of First Group Switch at Switch Position 3 First Group—2 3Fabric Port 2 Of Second Group Switch at Switch Position 1 First Group—24 Fabric Port 2 Of Second Group Switch at Switch Position 2 FirstGroup—2 5 Fabric Port 2 Of Second Group Switch at Switch Position 3First Group—2 6 Fabric Port 2 Of Second Group Switch at Switch Position4 First Group—3 1 Fabric Port 2 Of First Group Switch at Switch Position1 First Group—3 2 Fabric Port 2 Of First Group Switch at Switch Position2 First Group—3 3 Fabric Port 3 Of Second Group Switch at SwitchPosition 1 First Group—3 4 Fabric Port 3 Of Second Group Switch atSwitch Position 2 First Group—3 5 Fabric Port 3 Of Second Group Switchat Switch Position 3 First Group—3 6 Fabric Port 3 Of Second GroupSwitch at Switch Position 4

[0033] An alternative construction of an (n,m)-way switch is illustratedin FIG. 6B. In the specific exemplary alternative construction shown,the duplex switches are connected to form a (3,3)-way switch. As above,the switch, shown generally at 81, comprises a first group of duplexswitches 86 and a second group of duplex switches 87. Only the firstgroup of switches 86 can form duplex connections to any other port inthe module 81. The second group of switches 87 can only connect to theswitches of the first group 86. To effect duplex connection between theports of the first group of switches 86, an n-way switch 88 is used tointerconnect the fabric ports of the duplex switches of the first group86. The advantage of the alternative structure is that module 88 issimply an n-way switch. By choosing the cardinality of the individualmodules of the final overall switch architecture, the number ofdifferent part types used in the switch maybe reduced.

[0034] The architecture of the n-way reciprocal and (n,m)-way reciprocalmodules 60, 80 set forth in FIGS. 3 and 6A (6B), respectively, may becombined to emulate a Clos-like switching core. For comparison, athree-stage LM×LM Clos switching core is illustrated in FIG. 7. Asillustrated, there are three groups of switches 110, 115, and 120. Thefirst group of switches 110 is comprised of conventional, unidirectionalswitches 125 numbering M of the L×2L−1 switch type. The second group ofswitches 115 is comprised of conventional, unidirectional switches 130numbering 2L−1 of the M×M switch type. The third group of switches 120is comprised of conventional, unidirectional switches 135 numbering M ofthe 2L−1×L switch type.

[0035] The interaction of the switch groups 110, 115, and 120 andoperation of the resultant switching core are well-known. A significantproperty of the Clos switching structure is its recursive nature. Thisrecursive property allows a larger Clos switch to be formed from aplurality of smaller Clos switch structures.

[0036]FIG. 8 illustrates a strictly non-blocking core switch 150 havingfewer switches, yet having the same functionality as the LM×LM Closswitch of FIG. 7, except that such functionality is limited toreciprocal traffic. The switch core 150 is implemented using a pluralityof switching modules of the types described above in connection withFIGS. 3 (or 5) and 6. As illustrated, the switch core 150 employs twogroups of switching modules 155 and 160. The first group of modules 155is comprised of a plurality of (L,2L−1)-way modules 165 numbering M. The(L,2L−1)-way modules 165 are designed in accordance with the principlesof the (n,m)-way reciprocal switching module described above inconnection with FIG. 6, where n=L and m=2L−1. The second group ofmodules 160 is comprised of a plurality of M-way reciprocal switchingmodules 170 numbering 2L−1. The M-way modules 170 are designed inaccordance with the principles of the N-way reciprocal switching moduledescribed above in connection with FIG. 3A, where N=M.

[0037] The modules 165 of the first group of modules 155 are connectedto the modules 170 of the second group of modules 160 so that traffic atthe externally disposed I/O ports 180 handle reciprocal traffic in astrictly non-blocking manner. To this end, the fabric port at position jof the module 165 at position k of the first group of modules 155 isconnected to the fabric port at position k of the module 170 at positionj of the second group of modules 160. Examples of this interconnectionare set forth in Table 4. TABLE 4 Fabric Module Position Port PositionFabric Port Connection First Group—1 1 Fabric Port 1 Of Module of SecondGroup at Position 1 First Group—1 2 Fabric Port 1 Of Module of SecondGroup at Position 2 First Group—1 3 Fabric Port 1 Of Module of SecondGroup at Position 3 . . . . . . . . . First Group—1 L Fabric Port 1 OfModule of Second Group at Position L First Group—2 1 Fabric Port 2 OfSecond Group Switch at Switch Position 1 . . . . . . . . . First Group—2L Fabric Port 2 Of Second Group Switch at Switch Position L . . . . . .. . . First Group—M L Fabric Port M Of Second Group Switch at SwitchPosition L

[0038] Again, there are areas permutations that will work as long aseach module of the first group is connected to every module in thesecond group. The LM-way reciprocal core 150 appears to emulate a foldedversion of the Clos architecture of FIG. 7. The second group of modules160 of the LM-way reciprocal core is simply the left half of the middleClos stage 115 while the first group of modules 155 of the reciprocalcore 150 takes on the role of both outer stages 110 and 120 of the Closarchitecture.

[0039] Table 5 summarizes the complexity of the Clos switch core of FIG.7 and the switch core 150 in terms of the number of each elementalswitch type employed to implement the core, assuming each module inFIGS. 7 and 8 is implemented using 1×X switches and that the (n,M)-waymodules are implemented as shown in FIG. 6A: TABLE 5A LM-WAY LM × LMRECIPROCAL CORE NON-BLOCKING CLOS Duplex No. Of Simplex No. Of SwitchType Switches Switch Type Switches 1 × L-1 LM 1 × 3L-1 None 1 × 2L-1None 1 × 2L-1 2ML 1 × L M(2L-1) 1 × L 2M(2L-1) 1 × M-1 M(2L-1) 1 × M2M(2L-1)

[0040] The LM-way reciprocal core 150 employs half as many elementalswitches as the traditional Clos core. In the LM-way reciprocal core150, however, the 1×2L−1 switch type is replaced with a 1×3L−1 switchtype for implementing the (L,2L−1)-way modules 165. Although theelemental switches of the LM-way reciprocal core 150 are duplex switcheswhich are generally more costly than traditional switches, theincremental costs for such duplex switches will not generally exceed thesavings resulting from the reduced number of elemental duplex switchesthat are utilized. Further savings are realized from the presentinvention in terms of power and space requirements as well.

[0041] Similar efficiencies are realized when it is assumed that the(n,m)-way modules of switch 150 are implemented as shown in FIG. 6B.Such a comparison to the Clos switch is set forth in Table 5B. TABLE 5BLM-WAY LM × LM RECIPROCAL CORE NON-BLOCKING CLOS Duplex No. Of SimplexNo. Of Switch Type Switches Switch Type Switches 1 × 3L-1 None 1 × 3L-1None 1 × 2L-1 LM 1 × 2L-1 2ML 1 × L M(2L-1) + 1 × L 2M(2L-1) LM 1 × M-1M(2L-1) 1 × M 2M(2L-1)

[0042] As illustrated, the LM-way switch 150, when using thearchitecture of FIG. 6B, uses half as many 1×2L−1 and 1×M the switches,and approximately ¾ the number of 1×L switches. Although the elementalswitches used by the reciprocal core are duplex switches, and those ofthe Clos architecture are simplex switches, a cost savings can still berealized if the cost of a duplex switch is less than twice the cost ofthe simplex switch counterpart.

[0043] The traditional Clos architecture is a recursive architecture;i.e., a larger Clos core can be built using one or more smaller Closcores in the middle stages. Similarly, the LM-way reciprocal corearchitecture is also recursive. The LM-way reciprocal core 150 has thefunctionality of a LM-way duplex module. As such, it can be utilized asthe second stage module of a larger core. For example, a 256-portreciprocal core can be made by using three LM-way reciprocal cores of128 ports each as the second group of modules 160 in the architecture ofFIG. 8, and 128 (2,3)-way modules as the first group of modules 155.Such recursiveness facilitates ready expansion of an existing switchthereby allowing a user to upgrade their switching system withoutdisposing of existing hardware. Indeed, the LM-way reciprocal core 150may be modular—upgrading of the core merely comprising the addition ofone or more further modules. The first group of switches 155 of FIG. 8can also be used to expand an existing LM×LM non-reciprocal,non-blocking core, such as the one illustrated in FIG. 7. In suchinstances, the second group of switches 160 of FIG. 8 are replaced by aplurality of LM ×LM non-blocking cores, such as those illustrated inFIG. 7. This allows the (n,m)-way architectures to expand existingnon-reciprocal core architectures thereby eliminating the need topurchase new switching cores to replace the older, existing switchingcores.

[0044] Numerous modifications may be made to the foregoing systemwithout departing from the basic teachings thereof. Although the presentinvention has been described in substantial detail with reference to oneor more specific embodiments, those of skill in the art will recognizethat changes may be made thereto without departing from the scope andspirit of the invention as set forth in the appended claims.

1. A switch core comprising: a plurality of duplex switches; andinterconnection paths, through which communication signals canpropagate, interconnecting the plurality of duplex switches tofacilitate strictly non-blocking operation of the switch core forreciprocal traffic.
 2. A switch core as claimed in claim I wherein theswitch core is a non-square switch core.
 3. A switch core as claimed inclaim 1 wherein the switch core is a square switch core.
 4. A switchcore as claimed in claim 1 wherein at least some of the duplex switchesare optical duplex switches including optical signal routing components.5. A switch core as claimed in claim 4 wherein the optical signalrouting components include at least one of actuators, mirrors andlenses.
 6. A switch core as claimed in claim 4 wherein common signalrouting components of one or more of the optical duplex switches routeat least a pair of corresponding parallel signals, each transmitting asignal in a direction of signal travel opposite to the direction ofsignal travel of the other one of the corresponding parallel signals. 7.An N-way reciprocal switch core comprising: a plurality of duplexswitches numbering N of at least a 1×(N−1) switch type; interconnectionpaths, through which communication signals can propagate,interconnecting the plurality of duplex switches, so that each duplexswitch is connected to every other duplex switch by at least oneinterconnection path to thereby facilitate strictly non-blockingoperation of the switch core for reciprocal traffic.
 8. An N-wayreciprocal switch core as claimed in claim 7 wherein one or more of theplurality of duplex switches numbering N are of at least a 1×N switchtype.
 9. An N-way reciprocal switch core as claimed in claim 7 whereinthe interconnection paths connect each of the duplex switches to oneanother by exactly one interconnection.
 10. An N-way reciprocal switchcore as claimed in claim 8 wherein the interconnection pathsinterconnect the plurality of duplex switches so that each port Y ofeach switch X is connected to port X of switch Y except when X≠Y,wherein X is a number from 1 through N representing a switch position ofthe duplex switch vis-a-vis other duplex switches of the switch core andY is a number from 1 through N representing a position of a portvis-a-vis other ports of switch X, such interconnections proceeding fromX=1 until a single interconnection is provided between each of theduplex switches of the plurality of duplex switches.
 11. An N-wayreciprocal switch core as claimed in claim 10 wherein one or more of theduplex switches includes a loop-back port.
 12. An N-way reciprocalswitch core as claimed in claim 8 wherein one or more of the duplexswitches includes a loop-back port.
 13. An (n,m)-way switch corecomprising: a first group of duplex switches numbering n, each duplexswitch of the first group being of a 1×(n+m−1) type having n+m−1 ports;a second group of duplex switches numbering m, each duplex switch of thesecond group being of the 1×n type having n ports; interconnectionpaths, through which communication signals can propagate,interconnecting the first and second group of duplex switches, ports ofthe first group of duplex switches being connected to respective portsof every other duplex switch in the (n,m)-way switch core so that thefirst group of duplex switches is allowed to form duplex connections toany other duplex switch in the (n,m)-way switch core, each port of eachduplex switch of the second group of duplex switches being connectedonly to a respective port of a respective duplex switch of the firstgroup of duplex switches.
 14. An (n,m)-way switch core comprising: atleast one N-way switch; a first group of duplex switches; a second groupof duplex switches; interconnection paths, through which communicationsignals can propagate, interconnecting the at least one N-way switch andthe first and second group of duplex switches, ports of the first groupof duplex switches being connected to respective ports of the at leastone N-way switch and the second group of duplex switches so that thefirst group of duplex switches is allowed to form duplex connections toevery other switch in the (n,m)-way switch core, each port of eachduplex switch of the second group of duplex switches being connectedonly to a respective port of a respective duplex switch of the firstgroup of duplex switches.
 15. A switch core comprising: a plurality of(n,m)-way switches each comprising a first group of duplex switches; asecond group of duplex switches; interconnection paths, through whichcommunication signals can propagate, interconnecting the first andsecond group of duplex switches, ports of the first group of duplexswitches being connected to respective ports of every other duplexswitch in the (n,m)-way switch so that the first group of duplexswitches is allowed to form duplex connections to every other switch inthe (n,m)-way switch, each port of each duplex switch of the secondgroup of duplex switches being connected only to a respective port of arespective duplex switch of the first group of duplex switches, and atleast one Clos switch connected to and expanded by the plurality of(n,m)-way switches.
 16. An LM-way reciprocal switch core comprising: aplurality of (L,2L−1)-way reciprocal switches numbering at least M, eachincluding a plurality of interconnected duplex switches; a plurality ofM-way reciprocal switches numbering at least 2L−1, each including aplurality of interconnected duplex switches; interconnection paths,through which communication signals can propagate, interconnecting theplurality of (L,2L−1)-way reciprocal switches and the plurality of M-wayreciprocal switches to facilitate strictly non-blocking operation of theLM-way reciprocal switch core for reciprocal traffic.
 17. An LM-wayreciprocal switch comprising: a plurality of (L,2L−1)-way reciprocalswitches numbering M, each including a plurality of interconnectedduplex switches; a plurality of M-way reciprocal switches numbering2L−1, each including a plurality of interconnected duplex switches;interconnection paths, through which communication signals canpropagate, interconnecting the plurality of (L,2L−1)-way reciprocalswitches and the plurality of M-way reciprocal switches to facilitatestrictly non-blocking operation of the LM-way reciprocal switch core forreciprocal traffic.
 18. A reciprocal switch comprising: a plurality ofcascaded LM-way reciprocal switches, each LM-way reciprocal switchcomprising a plurality of (L,2L−1)-way reciprocal switches numbering atleast M, each including a plurality of interconnected duplex switches; aplurality of M-way reciprocal switches numbering at least 2L−1, eachincluding a plurality of interconnected duplex switches; andinterconnection paths, through which communication signals canpropagate, interconnecting the plurality of (L,2L−1)-way reciprocalswitches and the plurality of M-way reciprocal switches to facilitatestrictly non-blocking operation of the LM-way reciprocal switch core forreciprocal traffic.